The invention pertains to the field of chillers, and in particular to applying a variable speed drive to multiple chiller compressors.
A simplified typical air conditioning or refrigeration cycle includes transferring heat into a refrigerant, pumping the refrigerant to a place where heat can be removed from it, and removing the heat from the refrigerant. A refrigerant is a fluid that picks up heat by evaporating at a low temperature and pressure and gives up heat by condensing at a higher temperature and pressure. In a closed system, the refrigerant is then cycled back to the original location where heat is transferred into it. In a mechanical system, a compressor converts the refrigerant from a low temperature and low pressure fluid to a higher temperature and higher pressure fluid. After the compressor converts the refrigerant, a condenser is used to liquefy the fluid (gas) by cooling during the condensing part of the cycle. In operation, hot discharge gas (refrigerant vapor) from the compressor enters the condenser coil at the top, condenses into a liquid as heat is transferred to the outdoors. The refrigerant then passes through a metering device, such as an expansion valve, where it is converted to a low temperature, low pressure fluid before entering an evaporator.
A variable speed drive (VSD) is an electronic device connected between a power line and a device which can vary supply voltage and frequency. When applied to a refrigeration compressor, a VSD permits continually varying the compressor speed, giving an advantage of smooth operation and better partial-load performance. A VSD has a relatively high cost, which is a significant problem that has to be solved to allow using this technology in chiller markets. The cost of a VSD is proportional to absorbed current, which in turn is a function of the compressor capacity and operating conditions. In order to cover the entire range of application of VSD driven compressors, a VSD must be chosen which can provide the current corresponding to maximum load conditions, i.e., high condensing temperature and high suction. In reality, the compressor operates in such conditions only a few days each year. A need therefore exists to control the VSD in such a way as to make it practical to use a VSD with multiple compressors.
Briefly stated, the invention uses a variable speed drive (VSD) only on a range of capacities corresponding from zero to standard point conditions, and if conditions correspond to higher than rated point current, the VSD is disconnected and the compressor is connected directly on line. This approach allows using a smaller size VSD. At the same time, if there are several compressors operating in the same machine, only one VSD is needed. The VSD drives a first compressor from zero to the speed corresponding to the line frequency. Controls then switch the first compressor to direct on line (DOL) and disconnects the VSD from the first compressor. The VSD is then available to drive a second compressor. This process can be repeated through multiple compressors.
According to an embodiment of the invention, a method for controlling at least one compressor of a chiller system with a variable speed drive includes the steps of: (a) checking to see if additional compressor capacity is needed by the system; (b) ensuring the at least one compressor is running from the variable speed drive if additional compressor capacity is needed by the system; (c) monitoring a current in the variable speed drive; (d) closing an electronic expansion valve by a specified amount in the system if the current is greater than a specified percentage of a maximum current; (e) increasing a frequency of the variable speed drive by a specified amount if the current is less than or equal to the specified percentage of the maximum current; (f) checking to see if the additional compressor capacity needed by the system is satisfied by step (e); (g) checking whether the frequency of the variable speed drive is equal to a frequency of line power powering the variable speed drive, and if not, performing steps (c) through (f) again; and (h) transitioning the at least one compressor from variable speed drive to directly on line.
According to an embodiment of the invention, an apparatus for controlling at least one compressor of a chiller system with a variable speed drive includes means for checking to see if additional compressor capacity is needed by the system; means for ensuring the at least one compressor is running from the variable speed drive if additional compressor capacity is needed by the system; means for monitoring a current in the variable speed drive; means for closing an electronic expansion valve by a specified amount in the system if the current is greater than a specified percentage of a maximum current; means for increasing a frequency of the variable speed drive by a specified amount if the current is less than or equal to the specified percentage of the maximum current; means for checking to see if the additional compressor capacity needed by the system is satisfied by increasing the frequency of the variable speed drive; means for checking whether the frequency of the variable speed drive is equal to a frequency of line power powering the variable speed drive; and means for transitioning the at least one compressor from variable speed drive to directly on line if the frequency of the variable speed drive is equal to the frequency of the line power.